The size and aesthetics of many AM/FM receivers constrain the dimensions of their antennas and their ground planes to suboptimal limits.
For simplicity and performance, the desired size of a good-performing antenna is about one-half wavelength at the tuned frequency (e.g., a halfwave dipole). Although longer antennas can offer better antenna gain, their narrow beamwidths make them impractical for most applications. At FM frequencies, a half wavelength is about 1.5 meters, while for AM frequencies it is about 150 meters. Due to the presence of sensitivity-limiting ambient noise, which is greater at lower frequencies, it turns out that 1.5 meters is a sufficient length for both FM and AM receivers. A vertical quarter-wave whip antenna mounted over a large ground plane (e.g., a metal car body) exhibits performance similar to a halfwave dipole antenna, and affords good FM and AM reception in cars.
AM/FM receivers are available in many configurations, including automotive, tabletop, MP3 players, and cell phones. Smaller devices are typically characterized by poor signal reception, since halfwave and quarter-wave antennas are too large and impractical. An antenna size less than a half wavelength is considered electrically small. Electromagnetic interference (EMI) caused by antenna proximity to electronics, signal variability due to human body effects, and variable antenna orientation all have a significant impact on antenna performance. Because these effects are more pronounced at lower frequencies, many small devices have FM-only receivers.
The impedances of any antenna and receiver can theoretically be matched to achieve maximum power transfer. This can be realized by conjugate matching antenna and receiver impedances. Although it is well-known that conjugate matching of the antenna and receiver input impedances maximizes power transfer into the receiver, it is not necessarily optimum for receiver sensitivity. Conjugate impedance matching is practical for half wavelength antennas with relatively low reactance, and resistance nearly constant across the band, but this approach is not practical for the electrically small antennas used in many devices. The radiation and loss resistance of these antennas is very low, and the reactance is high. This high ratio of reactance to radiation resistance leads to matching techniques that maximize the voltage (not power) delivered to the receiver input.
The reactance of electrically small antennas can be tuned out using a resonant matching circuit. The Q of the resonant circuit thus formed must be held sufficiently high to increase the signal voltage to an acceptable level at the input to the receiver low noise amplifier (LNA). To achieve a sufficiently high Q, the receiver LNA must present a high parallel resistance to the antenna resonant circuit. Although higher values may be possible and could improve reception, a Q of about 30 is a practical goal for AM and FM reception.
Since a high Q circuit has a narrow bandwidth, it must be tunable across the AM or FM band as the receiver is tuned. This was common practice for AM tabletop superheterodyne receivers with internal loop antennas. The receiver mixed the RF input signal to a fixed intermediate frequency (IF) using a local oscillator (LO), mixer, and IF filter. The antenna for these receivers was either an air-loop (typical of older tube receivers) or a smaller ferrite-core loop antenna, both having similar characteristics. Since the inductance of this internal loop antenna was fixed and not significantly affected by external factors (such as the human body), a preselection filter was tuned along with the receiver LO to maintain a high Q resonant peak at the tuned frequency. This preselection filter, comprised of the loop antenna's inductance and a variable capacitance, also served as an image reject filter. Older superheterodyne receivers used a ganged-capacitor method to synchronize the preselection filter tuning with the LO tuning.
More modern receivers typically perform a similar function with varactor diodes acting as voltage-controlled capacitors. The voltage for the varactor diode of the preselection filter is derived from the tuning voltage of the LO varactor diode. Factory calibration is usually necessary for these receivers to accommodate component tolerances. Unfortunately, it is impractical for some modern receivers to utilize preselection filter tuning. The receiver IF is sometimes incompatible with this type of tuning, and calibration and consistency over operating temperature become impractical. Furthermore, external and portable antennas have time-varying impedance characteristics, making fixed factory calibration impossible.